Just when it goes into first in general. I put it down to being a low gear. It sounds a little bit like reverse, with the clutch pedal down or not...only it's not too loud. If you have the music on at a reasonable volume you don't hear it. As soon as it comes out of gear, or I go into second, the sound goes.
Other than that I have no issues at all, everything works perfectly. I had a gearbox on a 2007 Astra chew itself to bits, that sounded terrible! Needed a new gearbox.
Maybe it's meant to do that and I'm just being paranoid because I actually heard it.